This invention relates in general to endless metal belts, and in particular, to a process for strengthening the edges of endless metal belts.
Endless metal belts have been disclosed in the prior art for many purposes, including their use as drive members for continuously variable transmissions. When used in this manner, an endless metal belt must have certain properties and characteristics to operate efficiently.
The endless metal belt must be made of a material which is strong, exhibiting both a high fatigue strength which reduces the likelihood of failure from fatigue fracturing, and high compressive strength and tensile strength, which enable the belt to withstand the continuously varying demands imposed by the bending stresses inherent in the operation of the dual pulley system of the continuously-variable transmission. The belt material must be able to stretch without yielding and be flexible. It must be durable with a high wear resistance, because replacement requires the machine to be non-functional and is costly. The belt material must have high processability and be capable of being fashioned into a thin belt which can be manufactured to a high precision of circumferential length. In the event of multiple metal belts forming a continuously-variable transmission belt assembly, this high precision of circumferential length for each successive belt is especially critical to the formation of equal gaps between successive belts of the assembly. The multilayered belt assembly must have exacting tolerances with respect to the distance between the belts, as well as minimal friction between the belts. Each belt of a belt assembly must be capable of equal load sharing.
U.S. Pat. No. 3,604,283 to Van Doorne discloses a flexible endless member consisting of one or more layers of steel belts for use with a continuously-variable transmission, containing a driving mechanism which comprises a driving pulley with a V-shaped circumferential groove and a driven pulley with a V-shaped circumferential groove. The flexible endless member, which has chamfered (beveled) flanks, interconnects and spans the pulleys. The diameters of the pulleys automatically and steplessly can be varied with regard to each other in such a way that different transmission ratios can be obtained.
U.S. Pat. No. 4,661,089 to Cuypers discloses an endless metal belt for use with a continuously-variable transmission and a method for strengthening the belt to be less susceptible to the bending and tensile stresses to which a belt is subjected during the operation of a continuously-variable transmission. These stresses are often the source of hairline cracks which may result in early belt breakage. This method involves plastic deformation of the belt's edge zones by shot peening or rolling, thereby incorporating permanent compressive stresses in the belt edge zones.
Endless metal belts used for belt drives can be formed by several methods. One manufacturing method employs a "ring rolling method" similar to that described in Metals Handbook, 9th ed., wherein a metal, cylindrical tube is cut to a specified length and then an innermost layer is formed on the ring-rolling machine, making the ring wall thinner and the circumferential length longer. Subsequently, a number of additional layers, wherein the radius of each layer is slightly larger than that of the previously formed layer, may be similarly formed. The layers are then subjected to solution annealing in a vacuum furnace on a stainless steel cylinder, wherein the layers are rotated around two pulleys with tension in order to adjust the gap between the layers. After the dimensional correction, the layered belt is processed by precipitation-hardening (e.g., 490.degree. C. for 3 hours) and surface-nitriding. Finally, in order to improve lubrication ability between belts, surface profiling is performed.
U.S. Pat. No. 4,787,961 to Rush discloses a method of preparing a multilayered endless metal belt, wherein a tensile band set is formed from a plurality of separate looped endless bands in a nested and superimposed relation. The patent states that the bands are free to move relative to each other, even though the spacing between the adjacent bands is relatively small. At least one band may be formed by an electroforming process.
There is a need for a drive belt that is strong enough to carry a load which can also withstand stress, last longer and not be subject to premature failure.